The present invention relates to a filter-provided device with a filter in which a frequency value indicative of a passing frequency characteristic is set according to a value of a current inputted as a controlling signal, and specifically to a filter-provided device which is suitable for controlling a cut-off frequency of a low-pass filter of a high-frequency receiving device which receives a digital signal and demodulates the received digital signal.
Conventionally, a filter-provided device with a filter in which a frequency value indicative of a passing frequency characteristic is set according to a value of a current inputted as a controlling signal includes a high-frequency receiving device which is used in a tuner of a satellite broadcast etc. FIG. 5 is a circuit block diagram schematically showing a structure of a conventional high-frequency receiving device 100, and describes a case where the filter is a low-pass filter, and the frequency value indicative of the passing frequency characteristic is a cut-off frequency. In the high-frequency receiving device 100 of FIG. 5, when a high-frequency signal which is subject to a digital modulation is inputted, an amplifying circuit 102 amplifies a received signal, and further, an amplifying circuit 103 amplifies an output signal of the amplifying circuit 102 at a controlled gain. Next, the output signal of the amplifying circuit 103 is inputted to frequency changing circuits 104 and 105, and the frequency changing circuits 104 and 105 mix the signal inputted from the amplifying circuit 103 and a signal inputted from a phase shifting circuit 106, and outputs a base band signal.
Further, a PLL (Phase Locked Loop) circuit 108 to which a controlling signal S1 for determining a dividing ratio is inputted, controls a phase of a local oscillating signal which is generated in a local oscillating circuit 107, and is almost equal to a center frequency of a received frequency, based on a reference frequency signal generated in a reference signal oscillator 109, and the local oscillating signal of the local oscillating circuit 107 is inputted to the phase shifting circuit 106. The phase shifting circuit 106 generates a signal whose phase is shifted 90xc2x0 with respect to the local oscillating signal, and inputs the local oscillating signal of the original phase to one of the frequency changing circuits 104 and 105, and inputs the 90xc2x0 shifted signal to the other.
The output signals of the frequency changing circuits 104 and 105 are inputted to low-pass filters 110 and 111 respectively, and the low-pass filters 110 and 111 remove a high-frequency component of the inputted signals. The output signals of the low-pass filters 110 and 111 are inputted to AGC (Automatic Gain Control) amplifiers 112 and 113 respectively, and the AGC amplifiers 112 and 113 amplify the inputted signals at a controlled gain. The output signals of the AGC amplifiers 112 and 113 are inputted to low-pass filters 114 and 115 respectively, and the low-pass filters 114 and 115 remove interfering signals of adjacent channels or noise from the inputted signals at a cut-off frequency controlled by a controlling circuit 130 described later. The output signals of the low-pass filters 114 and 115 are inputted to A/D converters 118 and 119 after being amplified by amplifying circuits 116 and 117. The A/D converters 118 and 119 convert analog signals to digital signals so as to perform a demodulating process in a demodulating circuit 120 described later. The output signals of the A/D converters 118 and 119 are inputted to a demodulating circuit 120, and the demodulating circuit 120 demodulates the input signals which are subject to a digital modulation, and outputs a transport signal TS.
Further, a PLL circuit 122 to which a controlling signal S2 for determining the dividing ratio is inputted controls a phase of a local oscillating signal of a predetermined frequency which is generated in a local oscillating circuit 121, based on a reference frequency signal generated in a reference signal oscillator 123, and the local oscillating signal whose phase is controlled is inputted to the A/D converters 118 and 119 and the demodulating circuit 120. The A/D converters 118 and 119 and the demodulating circuit 120 are operated in accordance with the local oscillating signal as an operating signal. The operating signal becomes a sampling clock signal with respect to the A/D converters 118 and 119.
The low-pass filters 114 and 115 remove interfering signals of adjacent channels or noise, and function as an anti-aliasing of the A/D converters 118 and 119. The input signal has the modulating rate of several megabaud to dozens of megabaud. Thus, in order to make the low-pass filters 114 and 115 function effectively, it is required to set a cut-off frequency not to a fixed value, but to a suitable value according to the baud rate of the input signal. Further, when a cut-off frequency setting circuit is made up of a resistance and a capacitor in a circuit in an IC (Integrated Circuit), a variation of an IC process brings about a variation of the cut-off frequency of xc2x115% to xc2x120% . Thus, in a controlling circuit 130 shown in FIG. 5, a constant current source 137 is formed, and cut-off frequencies of the low-pass filters 114 and 115 are set by varying a constant current outputted from the constant current source 137 by the controlling circuit 131. In the constant current source 137, a signal generated based on a reference frequency signal which was generated in a reference signal oscillator 124 such as a crystal oscillator and has an accurate frequency (4 MHz in FIG. 5) is used as an input signal.
A circuit for generating the input signal of the constant current source 137 is a loop which includes a phase shift circuit 132, a mixer 133, a low-pass filter 134, a DC amplifying circuit 135, and a constant current source 136. The phase shift circuit 132 brings 90xc2x0 shift to a phase of a reference frequency signal inputted from the reference signal oscillator 124. The signal whose phase is shifted 90xc2x0 and the reference frequency signal outputted from the reference signal oscillator 124 are inputted to the mixer 133. The mixer 133 performs multiplication of the both signals, and the low-pass filter 134 removes a high-frequency component of an output signal of the mixer 133. Further, a DC amplifying circuit 135 amplifies an output signal of the low-pass filter 134. An output signal of the DC amplifying circuit 135 is inputted to the constant current sources 136 and 137, and the constant current sources 136 and 137 vary a current value based on the output current 135. A constant current of the constant current source 136 is inputted to the phase shift circuit 132, and the loop performs a control so that an output signal of the mixer 133 MIXout=0. Thus, the constant current sources 136 and 137 output constant currents according to a frequency of the reference frequency signal (4 MHz in FIG. 5).
The controlling circuit 131 includes a switch circuit based on a current mirror circuit, and is switched based on a controlling signal S3 which is inputted from outside according to a desired frequency, and adjusts a value of the constant current outputted from the constant current source 137 to a value according to a target cut-off frequency of the low-pass filters 114 and 115. An adjusting ratio is determined by the controlling signal S3, and is arbitrary. Further, a current outputted from the controlling circuit 131 is inputted as a controlling signal for controlling the cut-off frequency to the low-pass filters 114 and 115 respectively.
The low-pass filters 114 and 115 whose cut-off frequencies are controlled by the current outputted from the controlling circuit 130 arranged in this way are realized with a gmxe2x80xa2C filter made up of a transconductance amplifier (hereinbelow referred to as gm amplifier) and a capacitor, and a constant current controlling circuit. The phase shift circuit 132 of the controlling circuit 130 is a low-pass filter using the same type gmxe2x80xa2C filter as the low-pass filters 114 and 115, and the cut-off frequency is controlled by a master-slave system.
The controlling system of the cut-off frequency described above is generally used in a receiver which is arranged so that a current inputted to a transconductor circuit is varied by using a variable current source. The receiver is disclosed in Japanese Unexamined Patent Publication No. 257108/1998 (Tokukaihei 10-257108)(publication date: Sep. 25, 1998).
However, the switch circuit which makes up the controlling circuit 131 for controlling the cut-off frequencies of the low-pass filters 114 and 115 includes a large number of current mirror circuits and MOS-FETs. Thus, when a value of a constant current outputted from the constant current source 137 is adjusted to a value for the target cut-off frequency, a variation of an element characteristic and a current amplification factor hFE cause a large error between a current value and the target value. Also when a constant current of the constant current source 137 is multiplied equally by the controlling circuit 131, a signal passes via the switch circuit, so that the error is large. Especially, when a difference between a frequency of the reference frequency signal outputted from the reference signal oscillator 124 and the cut-off frequency is large, a signal passes via more circuits in the controlling circuit 131, so that an error of the current value becomes large. Thus, even when the cut-off frequency is controlled from outside of the low-pass filters 114 and 115, it is still impossible to set the cut-off frequency so accurately.
Further, in order to set the cut-off frequency so accurately, it is required to enlarge the size of a current switch circuit which controls the constant current source of the controlling circuit 131, so that power consumption is increased.
In controlling a cut-off frequency of a high-pass filter and a center frequency in a passband of a band pass filter, the foregoing problems can be brought about.
The object of the present invention is to provide a filter-provided device in which a frequency value indicative of a passing frequency characteristic, such as a cut-off frequency of a low-pass filter and of a high-pass filter and a center frequency of a pass band of a band pass filter, is set so accurately, and to provide a setting method of the passing frequency characteristic of the filter. Further, another object is to provide a filter-provided device which can reduce power consumption by a simple structure with the frequency value capable of being set so accurately, and to provide a setting method of the passing frequency characteristic of the filter.
The filter-provided device of the present invention, in order to achieve the foregoing objects, includes a filter whose frequency value indicative of a passing frequency characteristic is set according to a value of a current inputted as a controlling signal; a constant current source for inputting the current to the filter; a constant current controlling section for controlling the constant current source so as to output the current whose value corresponds to the frequency value indicative of the passing frequency characteristic which is set by the constant current source.
According to the structure, the constant current source controlling section controls the constant current source so that a current according to the frequency value indicative of the passing frequency characteristic which is to be set, such as a cut-off frequency of a low-pass filter, is outputted. The current outputted from the constant current source without performing a scale factor adjustment is inputted directly to the filter as a controlling signal, and the frequency value indicative of the passing frequency characteristic of the filter is set to a desired value. The scale factor adjustment of the value of the current is not performed, so that it is not required to provide a switch circuit for performing the scale factor adjustment. Thus, an error of a current value inputted to the filter is suppressed.
As a result, it is possible to provide the filter-provided device in which the frequency value indicative of the passing frequency characteristic of the filter can be set so accurately.
Further, besides the foregoing structure, in a case where an analog to digital converting section for converting an analog signal to a digital signal, and a sampling clock signal generating section for generating a sampling clock signal of the analog to digital converting section are provided, it is preferable that the constant current source controlling section generates a constant current reference signal including information of the frequency value indicative of the passing frequency characteristic based on the sampling clock signal, and controls the constant current source based on the constant current reference signal.
According to the structure, the constant current source controlling section generates the constant current reference signal including information of the frequency value indicative of the passing frequency characteristic of the filter based on the sampling clock signal generated by an existing sampling clock signal generating section, and controls the constant current source based on the constant current reference signal, so that it is possible to provide the filter-provided device which can reduce power consumption by a simple structure with the frequency value which indicates the passing frequency characteristic capable of being set so accurately.
Further, in a case where a signal processed by the filter is a small signal of a modulated rate, and the small signal is converted from an analog signal to a digital signal by the analog to digital converting section, it is possible to reduce the frequency of the sampling clock signal, so that power consumption can be reduced with the frequency reduced.
Further, besides the foregoing structure, it is preferable that the constant current source controlling section includes a frequency converting section for generating the constant current reference signal whose frequency is equal to the frequency value indicative of the passing frequency characteristic to be set, based on the sampling clock signal.
According to the structure, the constant current source controlling section equalizes the frequency of the constant current reference signal to the frequency value indicative of the passing frequency characteristic so as to include the frequency value indicative of the passing frequency characteristic in the constant current reference signal, and performs a control based on this, so that a current which corresponds to the frequency value indicative of the passing frequency characteristic is outputted. This enables the current of the constant current source to easily have a value according to the frequency value indicative of the passing frequency characteristic which is to be set.
Further, the filter-provided device according to another preferable embodiment of the present invention, in order to achieve the foregoing objects, includes a constant current reference signal generating section for varying the frequency of the constant current reference signal according to the frequency value indicative of the passing frequency characteristic which should be set; a constant current generating section which performs frequency to current conversion and outputs the constant current whose value corresponds to the frequency of the constant current reference signal; and a filter for controlling the passing frequency characteristic according to the value of the constant current.
According to the structure, the frequency to current conversion is performed with respect to the constant current reference signal whose frequency corresponds to the passing frequency characteristic which should be set, so that the constant current applied to the filter is generated. Thus, the constant current of the constant current generating section is inputted directly to the filter, unlike a structure in which the current is amplified at a scale factor according to the passing frequency characteristic to be set after generating a constant current whose current value is fixed.
Further, unlike a structure in which after judging what passing frequency characteristic should be set, the constant current is generated based on the judging result, it is possible to improve accuracy of the value of the constant current generated by the constant current generating section easily, even in a case where the frequency value indicative of the passing frequency characteristic is varied widely. Note that, it is possible to vary the frequency of the constant current reference signal generating section, for example by frequency dividing etc., more widely, and more accurately.
As a result, it is possible to set the constant current so that the value of the constant current corresponds to the frequency value accurately and easily, and to provide the filter-provided device which can set the frequency value indicative of the passing frequency characteristic of the filter so accurately.
Further, in a case where the filter-provided device includes an analog to digital converter for converting an analog signal which passed through the filter to a digital signal, it is preferable that the constant current reference signal generating section includes a sampling clock signal generating section for generating a sampling clock signal indicative of a sampling point of the analog to digital converter, and a signal generating section for generating the constant current reference signal based on the sampling clock signal.
According to the structure, since the constant current reference signal is generated based on the sampling clock signal, it is not required to provide another oscillator besides an oscillator for generating the sampling clock signal, when the constant current reference signal whose frequency corresponds to the frequency value which should be set is generated, so that it is possible to simplify the circuit structure.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.